1. Field of the Invention
The present invention relates to thermal bonding of nonwoven fabric, especially high elongation nonwoven fabric, and to calender rolls for producing such nonwoven fabric as well as to nonwoven fabrics and articles including nonwoven fabrics.
2. Description of Background Information
Nonwoven fabrics are typically made by making a web, and then thermally bonding the fibers together. For example, staple fibers can be converted into nonwoven fabrics using, for example, a carding machine, and the carded fabric can be thermally bonded. The thermal bonding can be achieved using various heating techniques, including heating with heated rollers, hot air and heating through the use of ultrasonic welding.
When heated rollers are utilized to bond the carded fabric, typically there is utilized one roll that is smooth and a calender roll that includes bonding spots thereon. The material to be thermally bonded is fed through a nip between the smooth roll and the calendar roll, with sufficient heat being applied to obtain thermal bonding of the material.
Conventional thermal bonding calender rolls, such as embossed calender rolls, are configured with a plurality of lands to provide bonding spots to provide maximum machine direction strength and cross direction strength, while preferably providing a soft feel or touch to the material, especially when forming the thermal bonded material or fabric from synthetic materials. Such thermal bonded fabric is often used to form nonwoven cover stock, which is typically used for hygiene products, such as a top sheet of a diaper. In such applications, one face or side of the cover stock material is placed in contact with a human body, for example, placed on the skin of a baby. Therefore, it is desirable that the face in contact with the human body exhibit softness.
The conventional processes utilize embossed thermal bonding calender rolls designed with a plurality of spaced lands that form bonding spots to provide a soft touch, as well as maximum machine direction and cross-direction strength. However, most of the prior art card-produced nonwoven fabrics do not have acceptable stretch characteristics.
There is therefore a need to produce nonwoven fabric having a high degree of elongation, especially with acceptable strength.
Accordingly, it is an object of the present invention to provide improved methods of manufacturing thermal bonded, nonwoven, fibrous web materials of high elongation, preferably with acceptable strength.
It is also an object of the present invention to provide a thermal bonding calender roll that will produce thermally bonded nonwoven material with high stretch or elongation.
The present invention provides a method of manufacturing thermally bonded fabric having high elongation, comprising forming a web of thermally bondable polypropylene fibers; passing the web over a heated calender roll having a patterned surface including a plurality of rows of lands, with each of the lands having an upper surface, with upper surfaces of lands in adjacent rows being staggered such that they do not overlap one another in a machine direction; and thermally bonding the fibers at bond points located about the upper surfaces of the lands such that fibers extending substantially in the machine direction will only have one bond point for adjacent rows.
The polypropylene fibers are preferably skin core polypropylene fibers.
The web can be provided as a carded fibrous web, or as a plurality of continuous filaments.
The elongation of the nonwoven fabric is at least about 120%, preferably greater than about 140%, more preferably greater than about 160%, even more preferably greater than about 180%, even more preferably greater than about 200%, even more preferably greater than about 250%, and most preferably greater than 300%.
The passing of the web over a heated calender roll can comprise passing the fibrous web between a pair of cooperating rolls in a calender nip, at least one of the pair of rolls having the patterned surface and comprising a cylindrical roll having an outer cylindrical surface including a plurality of lands; the lands being arranged on the outer cylindrical surface in a plurality of spaced apart rows; each of the lands comprising a tapered shape including a base, preferably substantially rectangular, tapering to an upper surface, which is preferably substantially rectangular; the upper surface including short sides oriented in the machine direction of about 0.036 cm to 0.066 cm in length, preferably about 0.043 cm to 0.059 cm, with a preferred value being about 0.051 cm, and long sides of about 0.071 cm to 0.133 cm in length oriented in a cross direction, preferably about 0.087 cm to 0.117 cm, with a preferred value being about 0.102 cm; adjacent rows of the lands being spaced apart a distance, with respect to the centers of the lands, of about 0.157 cm to 0.291 cm, preferably about 0.190 cm to 0.258 cm, with a preferred value being about 0.224; upper surfaces of lands in adjacent rows being staggered such that upper surfaces in adjacent rows do not overlap with one another in a machine direction providing a plurality of spaced apart bond points capable of bonding fibers extending substantially in the machine direction at only one bond point for adjacent rows; and the plurality of lands providing a bond area up to about 20%, preferably from about 5% to 20%, more preferably about 8% to 15%, even more preferably about 8% to 12%, with a preferred value being about 11%
The upper surfaces of the lands can be substantially rectangular. Moreover, the bases can overlap in the machine direction in adjacent rows.
Centers of the lands in each row are spaced apart by a distance of about 0.142 cm to 0.264 cm, preferably about 0.173 cm to 0.233 cm, with a preferred value being about 0.203 cm, and centers of the lands in adjacent rows are spaced apart by a distance of about 0.071 cm to 0.133 cm, preferably about 0.087 cm to 0.117 cm, with a preferred value being about 0.102 cm. The lands are constructed and arranged such that a straight fiber lying at an angle of greater than 15% with respect to the machine direction, more preferably about 35xc2x0 to 55xc2x0, and even more preferably 45xc2x0, is positioned in a space between the lands, whereby the fiber is not be bonded.
The lands are preferably of a truncated pyramidal shape, preferably having substantially rectangular bases with short sides about 0.075 cm to 0.139 cm in length oriented in the machine direction, preferably 0.091 cm to 0.123 cm, with a preferred value being about 0.107 cm, and long sides about 0.111 cm to 0.205 cm in length oriented in the cross direction, preferably about 0.134 cm to 0.182 cm, with a preferred value being about 0.158 cm, and tapered side walls extending between the base and the upper surface at an angle a of about 14xc2x0 to 35xc2x0, preferably about 17xc2x0 to 26xc2x0, more preferably about 17xc2x0 to 23xc2x0, with a preferred value being about 20xc2x0.
The lands can have a height of about 0.053 cm to 0.099 cm, preferably about 0.065 cm to 0.087 cm, with a preferred value being about 0.076 cm.
There is also provided a method of manufacturing thermally bonded fabric having high elongation, comprising forming a web comprising thermally bondable skin-core polypropylene fibers; passing the web over a calender roll having a patterned surface including a plurality of rows of lands, with each of the lands having an upper surface, with upper surfaces of lands in adjacent rows being staggered such that they do not overlap one another in a machine direction; applying heat and pressure to the fibrous web at a location of the calender roll to thermally bond the fibers at bond points located about the upper surfaces of the lands such that fibers extending substantially in the machine direction will only have one bond point for adjacent rows.
The present invention is also directed to the calender roll and to polypropylene fabrics formed by the methods.